The production of many parts and products is done by multi-stage manufacturing systems. At each stage, certain production equipment performs a particular manufacturing operation that may include several tasks. A task may be, for example, drilling a hole or welding a spot, or inserting a pin in a hole. The partly-finished part is transferred from one stage to the next via a material transport system, such as a conveyor, a robot, an autonomous guided vehicle (AGV), an overhead gantry, or by people. This multi-stage production method is typical to medium and high-volume manufacturing of a variety of parts and products ranging from engines, pump housings, appliances, cars, to microprocessors. The specific production equipment in the system at each stage depends on the production domain. In machining operations, for example, the production equipment may be a machine tool or an inspection station. In assembly, the equipment may be a welding robot, and in microprocessor production—a chemical process.
Typically, these multi-stage manufacturing systems are built as a sequential, serial line. If the required volume of parts is higher (i.e., larger system capacity is needed), then a second serial line may be added. A recent survey conducted in Europe and the US by the NSF Engineering Research Center for Reconfigurable Manufacturing Systems at the University of Michigan reveals that industries are “Very Dissatisfied” with the large floor space that multi-stage systems occupy. Therefore, the reduction in floor space is an important challenge to the manufacturing industry.
Another challenge is how can one scale up a system production capacity in a cost-effective, rapid method when the market demand increases. Traditional machining systems, for example, are of two types: Dedicated and flexible. The dedicated systems include serial (sequential) production lines consisting of dedicated machines that are designed to produce only one particular part at very large quantities. The dedicated machines produce parts at a high production rate, which is achieved by performing on the part several tasks simultaneously. In other words, a dedicated machine uses parallel tools to drill or tap several holes simultaneously or cut surfaces simultaneously. For example, a dedicated machine can drill twenty holes of different diameters simultaneously by using a multi-tool spindle head, which enhances dramatically the productivity of the machine.
By contrast to dedicated systems, flexible manufacturing systems (FMS) can produce a variety of parts on the same system. The production equipment in FMS for machining includes mainly computerized numerically controlled (CNC) machine tools, each equipped with only one cutting tool (e.g., a drill of a particular diameter, or a milling cutter) whose motions are controlled by a computer. Compared to the dedicated machines, the CNC machines are slow. To drill twenty holes, for example, the drilling tool is moved to a point located above the first hole-location, then moved down to the fist hole location to drill the first hole, then retracted, and moved to the next hole location—a sequence of tasks that has to be repeated twenty times to drill the twenty holes. This is a much slower operation than that may be performed with a twenty-tool spindle-head on the dedicated machine. The CNC machine, however, is flexible because its cutting tool can be automatically changed, and a new-part program that controls the tool motions can be easily loaded into its computer. This flexibility allows using the system to produce new type of parts when needed, and also to produce several different types of parts on the same day using the same CNC machine. Thus, the CNC machines are critical enablers that make the whole machining system flexible.
Another challenge relates to in-process inspection of parts. Currently machining systems utilize two types of dimension inspection:
(1) In-process measurement by dedicated mechanical gauges that provide a binary “Good/Not-Good” (or “Go/No-Go”) output. Each time that a different type of parts is produced, these gauges have to be changed. These gauges are limited to measuring a small number of dimensions, and cannot measure such features as surface flatness or parallelism of two surfaces; and
(2) Measurements by Coordinate Measuring Machines (CMM) that are usually placed in a separate room. The finished parts are taken to the CMM for inspection. The CMM includes a one-dimensional measurement touch-probe that moves from one inspected point to the next while the coordinates of each point are measured. This is a slow process, such that it may take two to three hours for a part such as a cylinder head of a car engine to be inspected. During the inspection time, the system continues to produce parts at a rate of about 100 per hour. If, after three hours of inspection, a defected part is found, then some 200-300 parts have to be scrapped.
One solution to this problem may be provided by a Reconfigurable (in-process) Inspection Machine (RIM), which is described in U.S. Pat. No. 6,567,162, co-owned by the assignee, The Regents of the University of Michigan, and incorporated herein by reference in its entirety. It is still desirable, however, to integrate the RIM into the manufacturing system such that the production flow is not interrupted when the RIM requires maintenance or repairs. It is, therefore, not advisable to install the RIM in series with the manufacturing equipment.